This invention pertains to circuits which can be used with telephone lines to detect on- and off-hook conditions of a telephone and thereby control other apparatus in response to such conditions. For example, a source of audio in a room can reduce a caller's or a listener's ability to hear a telephone call after the telephone receiver has been taken off the hook in order to answer the call.
In the event that a circuit is to be provided which automatically causes such an audio source to be muted, the circuit must be undetectable so as to prevent any damage to, or degradation of, telephone service. Moreover, such a circuit must not interfere with the audio source when the telephone is off- or on-hook. Finally, it would be desirable for such a circuit to remain uninfluenced by any AC component across the telephone line, so as to prevent improper alternating muting of the audio source.